The ignition timing of an engine is determined based on a crankshaft position and a camshaft position. For example, a camshaft of a four-stroke engine attains one rotation for every two rotations of a crankshaft. Therefore, cylinder identifying information is provided within one rotation of the camshaft and ignition timing information is provided in one rotation of the crankshaft.
Conventional position detecting devices use MREs for a determination of a rotor position. In the devices, a bias magnetic field is projected by a bias magnet toward a rotor. The direction of the bias magnetic field changes as the rotor position changes associated with rotation of the rotor. Therefore, the rotor position is determined based on the changes of the direction of the bias magnetic field. However, immediately after the rotor starts rotating, an accurate rotor position cannot be determined until the direction of the bias magnetic field changes. Thus, the first cylinder determination based on the rotor position cannot be performed, and an ignition is not performed at the first ignition timing.
To solve this problem, a position detecting device 1 that detects a rotor position even when the rotor is at a halt is invented and disclosed in JP-A-11-237256. The position detecting device 1 includes two MRE bridges as shown in FIG. 13. The MRE bridges 6, 16 are composed of a first pair of MREs 4, 5 and a second pair of MREs 17, 18 connected in series, respectively.
The MREs 4, 5, 17, 18 are arranged so that their sensing axes are at angles of 45° and −45° with respect to a magnetic center of a bias magnetic field. With this configuration, changes in voltages at respective connecting points of the first pair and the second pair, in response to changes of the magnetic field direction, become more significant.
Output voltages of the MRE bridges 6, 16 are inputted to and amplified by the differential amplifier circuit 20. The differential output of the circuit 20 corresponds to a deflection angle of the bias magnetic filed. The MRE bridges 6, 16 are positioned off magnetic center of the bias magnet field. As a result, different output is obtained in each case that a gear-tooth of the rotor is adjacent to or away from the bridges 6, 16. Therefore, the rotor position is determined even when the rotor is at a halt.
However, the output of the circuit 20 varies according to a gear-teeth shape, which changes an air gap between the rotor and the MRE bridges 6, 16. To perform accurate determination of the rotor position, a threshold is provided for binarizing the output of the circuit 20. The threshold is defined based on a minimum point of air gap (AG) characteristic curves. At the minimum point, the output of the circuit 20 is always equal regardless of the size of the air gap if the rotor position is equal as shown in FIG. 15.
In the device 1, the output at the minimum point varies according to the gear-teeth shape. As a result, an accuracy of the rotor position determination decreases if a single threshold is used. To provide accurate rotor position determination, different threshold values need to be set for rotors having different gear-teeth shapes. This creates heavy workload.